The present invention relates to an engine knock detecting apparatus for use in an ignition timing controller of internal combustion engine or a like device, which controller adjusts the ignition timing. of the engine so that a predetermined degree of knocking occurs. More specifically, the present invention relates to a knock detector which detects engine knock by sensing vibrations outside the cylinder due to internal pressures therein.
It is well known that there is a close relationship between ignition timing and the internal pressure of a cylinder. In the ordinary state of combustion without any knock, no higher harmonic appears in the internal pressure of the cylinder. The term "higher harmonics" is used here to mean vibration components having frequencies generally ranging between 5 KHz and 10 KHz and between 11 KHz and 13 KHz, the frequency being determined by the bore diameter of the cylinder and the sonic velocity. This higher harmonic is caused by a detonation in the combustion.
As knocking takes place, these higher harmonics begin to appear as the internal pressure of the cylinder is increased to the level near the maximum pressure. These higher harmonics are observed at the outside of the cylinder as an abnormal vibration or noise. A full analysis of the signal representing the internal pressure or the vibration or noise observed at the outside of the cylinder reveals the following. The initial symptom of knocking (trace knocking) is observed at an engine crank angle corresponding to the maximum pressure in the cylinder. As the degree of knocking advances to the state of light knock and then to the state of heavy knock, the crank angle at which the higher harmonics are generated is shifted to the advanced side, i.e. toward the ignition angle, from the crank angle at which the maximum pressure is observed.
It is, therefore, possible to remarkably increase the efficiency of the engine by a suitable feedback control of the ignition timing upon minute sensing of the vibration and noise appearing at the outside of the cylinder.
At the present stage, however, there is no apparatus which can accurately detect the state of knock as the factor to be fed back and operate stably under the severe operating conditions which the vehicle must undergo.
Two types of detecting apparatuses for this purpose are known: a commercially available detecting apparatus making use of a piezoelectric accelerometer and a resonance type apparatus which is now under discussion and development by the present inventors.
The piezoelectric accelerometer type apparatus has a resonance point which is usually at a frequency range above the frequency range at which engine knock takes place, and exhibits a substantially flat detection characteristic for the frequencies below the knock frequency. This type of apparatus, therefore, will be referred to as "non-resonance type" hereinunder. In contrast, the resonance type detecting apparatus has a resonance point at the frequency level of the knock, so that it has a high sensitivity in the frequency region around the resonance point as compared with other frequencies. However, due to a pecularity in the resonance, the range of detectable frequencies is reduced as the peak Q of the resonance becomes higher.
The sensor of the non-resonance type suffers an inferior S/N ratio due to the vibration noise of the engine, so that it cannot be practically used in detecting knocking at high engine speed, although it theoretically provides a wide range of detectable frequencies. To the contrary, the resonance type apparatus has much superior detection performance due to a high S/N ratio and high sensitivity, but the Q value is so high that the range of sensitivity is impractically small. In consequence, the resonance type detecting apparatus may fail to detect knocking when the resonance point is shifted or when the knocking frequency is changed due to a change in the state of combustion. This feature is quite disadvantageous because, while knock takes place over a wide range of frequencies, the detection apparatus cannot provide uniform detection characteristics over the entire frequency range and, therefore, can detect frequencies of only a narrow range. In spite of this fact, the resonance type detecting apparatus still has much superior detection characteristics as compared with the non-resonance type. For attaining a higher detection accuracy, therefore, it is desirable to obtain a trapezoidal detection characteristic curve having wide range of detectable frequencies at a cost of a lowered Q value.
Generally, there is a close relationship between the Q value and the attenuation characteristic in response to an impulse. More specifically, the attenuation time increases as the Q value increases, while the attenuation time is shortened as the Q value becomes smaller. Usually, the judgment as to whether knocking is taking place or not is made through the comparison of the output from the detecting apparatus obtained when knocking is taking place in the engine with the integrated value of the output from detecting apparatus as obtained when knocking is not taking place, i.e. with the output corresponding to the vibration noise of the engine. Therefore, if the attenuation time is long due to the high Q value, the noise component becomes great to deteriorate the S/N ratio. In other words, it is desirable that the output from the detecting apparatus is attenuated without delay after knocking. The reduction of Q value is preferred also from this point of view.